The present application is directed to load ballasts and, more particularly, to a novel protection circuit for reducing load current in the event of arc-like conditions in a capacitive ballast or associated load.
It is known to control loads by means of a powercontrolling ballast. Typically, a variable ballasting impedance is placed in series with a load, such as in the capacitive ballast described and claimed in co-pending U.S. patent application Ser. No. 379,393, filed May 18, 1982, now U.S. Pat. No. 4,447,764, assigned to the assignee of the present invention and incorporated herein in its entirety by reference. A capacitive ballast may be used with load resistance control circuitry, such as described and claimed in co-pending U.S. patent application Ser. No. 382,875, filed May 28, 1982, now U.S. Pat. No. 4,421,993, also assigned to the assignee of the present invention and incorporated herein in its entirety by reference. However, the load resistance control circuitry does not protect the capacitive ballast, or the associated load, from adverse effects of an arc, or arc-like, condition occurring in the power circuit. These arcs may be obtained from a poor connection to the energizing source or improper action in a circuit-controlling switch and the like. Typically, if an arc-like condition has occurred during a power source line cycle, the arc-like condition will continue until a natural zero crossing of the line and load current, at which natural zero crossing point the arc will extinguish but will leave a highly ionized gap. In a capacitive ballast, the capacitor voltage lags the current substantially by 90.degree., whereby the capacitor voltage is at a line peak at the moment the arc extinguishes, and tends to remain at the peak level. When the line voltage falls from this peak level in a sinusoidal manner, the potential across a highly ionized gap, caused by the arc-like condition, increases until a breakdown potential is reached, at which potential the arc is itself reestablished. The energy stored in the capacitor of the ballast is suddenly dumped into the load and is limited only by the load resistance. Therefore, with a reignition time of about one-half cycle, maximum energy transfer to the load occurs and the load, or a portion of the ballast, may be instantly destroyed. It is therefore highly desirable that these destructive currents be eliminated or reduced by detection of the occurrence of an arc, and by providing suitable means for reducing the energy available from the ballasting capacitance under such conditions.